1. Field of the Invention
The present invention relates to packet relay apparatuses and packet relay methods, and particularly to a packet relay apparatus for performing packet relay control and a packet relay method for performing packet relay control.
2. Description of the Related Art
Many applications such as real-time voice processing for telephone communication, video, and the like, remote control by a robot, and remote medical practice (surgery) have been developed in recent years. When information used in these applications is packetized for packet communication, missing data or a delay of just one single packet would have a great influence on the speech quality or the reproduction quality. Accordingly, highly-reliable communication with a good quality of service (QoS) has been required, and traffic control that can protect application operations from failure has been desired.
Conventional traffic control can be broadly divided into priority control and bandwidth control. In priority control, the priorities of packet types are determined, and the packets are sent in order of priority.
In bandwidth control, a bandwidth that can be used by particular packets (packets having high real-time characteristics) is determined among the entire bandwidth of a transmission path, and the other packets are sent in the remaining bandwidth under priority control. For example, if the entire bandwidth is 100 Mbps, a bandwidth of 30 Mbps is always reserved for video packets, and the remaining bandwidth of 70 Mbps is used to send non-video packets in order of priority.
One conventional traffic control technology in packet communication provides a plurality of relay channels in correspondence with packet attributes and sends each packet from the relay channel provided to the corresponding packet attribute (refer to Japanese Unexamined Patent Application Publication No. Hei-10-13434 (paragraph numbers 0017 to 0028, FIG. 1), for instance).
If network congestion occurs, or if packet processing in an apparatus takes much time, the conventional traffic control as described above causes packets to be left unsent in an internal buffer (queue) of the apparatus and to be sent from the buffer in a best-effort manner (sent at the maximum available rate) when the bandwidth becomes available. When this burst transfer occurs, a receiving apparatus could fail to receive a packet.
Networks such as the Ethernet (registered trademark) perform transmission control by using the closed clock of the apparatus (asynchronous clock for transmission and reception), so that there occurs a difference in transmission clock between the transmitting apparatus and the receiving apparatus. A comparison between the clock of the transmitting apparatus and the clock of the receiving apparatus may sometimes reveal a frequency difference of about ±100 ppm.
If an increasing number of packets left unsent in the buffer are sent at once on the transmission path in a best effort manner, increasing the packet traffic to nearly the maximum capacity, the receiving side could not receive all of those packets and could miss some packets because of the difference between the transmission and reception clock timings.
In addition, the conventional receiving apparatus requires a large buffer to stand the burst transfer, regardless of whether a packet is lost or not, so that a large-size apparatus is needed.
If a burst transfer occurs, the packets arrive at the receiving side at irregular intervals, so that the time of reproduction processing on the receiving side would depend on each packet, causing the reproduced video to be disturbed or the sound to be interrupted, which makes the user feel unusual.